The present invention relates to concrete crack repair and, more particularly, to a method and apparatus for low-profile concrete post-tensioning when restoring tensile strength in a cracked concrete section.
Concrete serves as the primary material in the construction of foundations, columns, beams and load-carrying slabs. Unfortunately, concrete can crack during its lifetime for several common reasons, such as shrinkage, thermal expansion and contraction, low tensile strength and the like. Non-reinforced cracks or fractures cannot transfer loads from one slab section to the adjacent slab section, and so is in need of repair.
Repair of fractured concrete, to be successful, often needs additional steel placed in tension to provide structural stability by transferring the tensile load. Conventional methods for structural crack repair options included: doweling, epoxy injection, adding “U” clip reinforcement, and post-tensioning.
Doweling consists of drilling holes and anchoring straight steel dowels across the crack. The straight steel is anchored with epoxy to solid areas of reinforced concrete. However, doweling is often impractical for typical slab thicknesses of 8 inches or less. Also, doweling is known to be not very effective for restoring tension in flexural members.
Epoxy injection makes the injected crack stronger than the adjacent concrete. If active cracks are injected, new cracks can form next to or far away from the repaired crack unless one employs sufficient amounts of tensioned reinforcing. Moreover, often times the epoxy and concrete interface separates—bond separation—under loading. As a result, bond separation of the epoxy, like the original cracking it aimed to repair, cannot help to bear the load across the fracture.
By adding “U” clip reinforcement, cracked concrete is repaired with reinforcing bars or metal “U” clips. Concrete crack repair by this option is done by drilling holes or slots across the crack plane. The reinforcing bars are not placed in tension and are glued in across the crack in the slot or drilled holes. Although adding reinforcing over a crack adds strength to the region, the crack has to re-form before it resists tension. Also, with many conventional and employable methods of repair, cracking had to occur over the repaired area before added reinforcement could resist movement. Although post-tensioning is often recommended as the best choice to repair fractured concrete, it often cannot be applied since the tensioning mechanism would reside above the concrete surface and inhibit future use of the slab. Post-tensioning is a good concrete crack repair solution when a major portion of a member must be strengthened or when a crack must be closed. Post-tensioning—strands or bars are used to apply compressive force to the ailing member. Employing this method calls for adequate anchorage for the post-tensioning steel as well as to balance the effect of the tensioning force and eccentricity on stresses in the structure. As a result, utilizing this method often involves a very large (high profile) mechanism that significantly protrudes from the surface of the fractured member.
As can be seen, there is a need for an improved low-profile method and apparatus for repairing or improving load transfer across cracks in concrete as well as for resisting future cracking.